Derivative Instruments - Derivatives Gains and Losses (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Commodity derivatives | Cost of goods sold
Derivative Instruments, Gain (Loss) [Line Items]
Total income (losses)		$ 7.8	$ (71.2)	$ (46.4)
Derivatives designated as hedging instruments: | Commodity derivatives | Cost of goods sold
Derivative Instruments, Gain (Loss) [Line Items]
Realized losses reclassified out of OCI on commodity derivatives designated as cash flow hedging instruments		(1.7)	(38.6)	(27.8)
Gains recognized on commodity derivatives due to cash flow hedging ineffectiveness		0.9	0.5	3.1
Derivatives not designated as hedging instruments: | Cost of goods sold
Derivative Instruments, Gain (Loss) [Line Items]
Unrealized gain (losses) on cash flow hedging instruments		32.1	(13.0)	(34.2)
Unrealized gains (losses) on cash flow hedges that have matured but not settled		8.1	4.6
Derivatives not designated as hedging instruments: | Commodity derivatives | Cost of goods sold
Derivative Instruments, Gain (Loss) [Line Items]
(Losses) gains on derivatives not designated as hedging instruments	[1]	0.9	(33.1)	(21.7)
Derivatives not designated as hedging instruments: | Commodity derivatives | Other operating income (expense)
Derivative Instruments, Gain (Loss) [Line Items]
(Losses) gains on derivatives not designated as hedging instruments	[1],[2]	$ 7.7	$ 0.0	$ 0.0

[1] osses) on commodity derivatives that are economic hedges but not designated as hedging instruments include unrealized gains (losses) of $32.1 million, $(13.0) million and $(34.2) million for the years ended December 31, 2018, 2017 and 2016, respectively. Of these amounts, approximately $8.1 million and $4.6 million for the years ended December 31, 2018 and 2017, respectively, represent unrealized gains where the instrument has matured but where it has not cash settled as of period end, excluding the reversal of prior period settlement differences. Derivative instruments that have matured but not cash settled at the balance sheet date continue to be reflected in derivative assets or liabilities on our balance sheet.

[2] See separate table below for disclosures about "trading derivatives."